p53 proteinA tumor suppressor protein with critical functions in normal cells. A mutation in the gene that encodes it, p53, can result in loss of control over cell division and thus cancer.
paedogenesis(Gr. pais, child, + genes, born). Reproduction by immature or larval animals caused by acceleration of maturation. Progenesis.
paedomorphosis(Gr. pais, child, + morphe, form). Retention of ancestral juvenile features in later stages of the ontogeny of descendants.
pair bondAn affiliation between an adult male and an adult female for reproduction. Characteristic of monogamous species.
pallium(L. mantle). Mantle of a mollusc or brachiopod.
pangenesis(Gr. pan, all, + genesis, descent). Darwin's hypothesis that hereditary characteristics are carried by individual body cells that produce particles that collect in the germ cells.
papilla(L. nipple). A small nipplelike projection. A vascular process that nourishes the root of a hair, feather, or developing tooth.
papula(L. pimple). Respiratory processes on skin of sea stars; also, pustules on skin.
parabiosis(Gr. para, beside, + biosis, mode of life). The fusion of two individuals, resulting in mutual physiological intimacy.
paramylon bodies(Gr. para, beside, + mylos, mill, grinder). Organelles containing the starch-like substance paramylon; in some algae and flagellates.
paraphyly(Gr. para, beside, + phyle, tribe). The condition that a taxon or other group of organisms contains the most recent common ancestor of all members of the group but excludes some descendants of that ancestor; contrasts with monophyly and polyphyly
parapodium(Gr. para, beside, + pous, podos, foot). One of the paired lateral processes on each side of most segments in polychaete annelids; variously modified for locomotion, respiration, or feeding.
parasitism(Gr. parasitos, from para, beside, + sitos, food). The condition of an organism living in or on another organism (host) at whose expense the parasite is maintained; destructive symbiosis.
parasympathetic(Gr. para, beside, + sympathes, sympathetic, from syn, with, + pathos, feeling). One of the subdivisions of the autonomic nervous system, whose fibers originate in the brain and in anterior and posterior parts of the spinal cord.
parenchyma(Gr. anything poured in beside). In lower animals, a spongy mass of vacuolated mesenchyme cells filling spaces between viscera, muscles, or epithelia; in some, cell bodies of muscle cells. Also, the specialized tissue of an organ as distinguished from the supporting connective tissue.
parenchymula(Gr. para, beside, + enchyma, infusion). Flagellated, solid-bodied larva of some sponges.
parietal(L. paries, wall). Something next to, or forming part of, a wall of a structure.
parthenogenesis(Gr. parthenos, virgin, + L. from Gr. genesis, origin). Unisexual reproduction involving the production of young by females not fertilized by males; common in rotifers, cladocerans, aphids, bees, ants, and wasps. A parthenogenetic egg may be diploid or haploid.
pathogenic(Gr. pathos, disease, + N.L. genic, giving rise to). Producing or capable of producing disease.
PCRSee polymerase chain reaction
peck orderA hierarchy of social privilege in a flock of birds.
pecten(L. comb). Any of several types of comblike structures on various organisms, for example, a pigmented, vascular, and comblike process that projects into the vitreous humor from the retina at a point of entrance of the optic nerve in the eyes of all birds and many reptiles.
pectines(L. comb, pl. of pecten). Sensory appendage on abdomens of scorpions.
pectoral(L. pectoralis, from pectus, the breast). Of or pertaining to the breast or chest; to the pectoral girdle; or to a pair of horny shields of the plastron of certain turtles.
pedal lacerationAsexual reproduction found in sea anemones, a form of fission.
pedalium(L. pedalis, of or belonging to the foot). Flattened blade at the base of the tentacles in cubozoan medusae (Cnidaria).
pedicel(L. pediculus, little foot). A small or short stalk or stem. In insects, the second segment of an antenna or the waist of an ant.
pedicellaria(L. pediculus, little foot, + aria, like or connected with). One of many minute pincerlike organs on the surface of certain echinoderms.
pedipalps(L. pes, pedis, foot, + palpus, stroking, caress). Second pair of appendages of arachnids.
peduncle(L. pedunculus, dim. of pes, foot). A stalk. Also, a band of white matter joining different parts of the brain.
pelage(Fr. fur). Hairy covering of mammals.
pelagic(Gr. pelagos, the open sea). Pertaining to the open ocean.
pellicle(L. pellicula, dim. of pellis, skin). Thin, translucent, secreted envelope covering many protozoa.
pelvic(L. pelvis, a basin). Situated at or near the pelvis, as applied to girdle, cavity, fins, and limbs.
pelycosaur(Gr. pelyx, basin, + sauros, lizard). Any of a group of carnivorous Permian synapsids distinguished by powerful jaws, stabbing teeth, and a large skin-covered sail on the back.
pentadactyl(Gr. pente, five, + daktylos, finger). With five digits, or five fingerlike parts, to the hand or foot.
pentamerous symmetry(Gr. pente, five, + meros, part). A radial symmetry based on five or multiples thereof.
peptidase(Gr. peptein, to digest, + ase, enzyme suffix). An enzyme that breaks down simple peptides, releasing amino acids.
peptide bondA bond that binds amino acids together into a polypeptide chain, formed by removing an OH from the carboxyl group of one amino acid and an H from the amino group of another to form an amide group-CO-NH-
perennibranchiate(L. perennis, throughout the year, + Gr. branchia, gills). Having permanent gills, relating especially to certain paedomorphic salamanders.
pericardium(Gr. peri, around, + kardia, heart). Area around heart; membrane around heart.
periostracum(Gr. peri, around, + ostrakon, shell). Outer horny layer of a mollusc shell.
peripheral(Gr. peripherein, to move around). Structure or location distant from center, near outer boundaries.
periproct(Gr. peri, around, + proktos, anus). Region of aboral plates around the anus of echinoids.
perisarc(Gr. peri, around, + sarx, flesh). Sheath covering the stalk and branches of a hydroid.
perissodactyl(Gr. perissos, odd, + daktylos, finger, toe). Pertaining to an order of ungulate mammals with an odd number of digits.
peristalsis(Gr. peristaltikos, compressing around). The series of alternate relaxations and contractions that serve to force food through the alimentary canal.
peristomium(Gr. peri, around, + stoma, mouth). Foremost true segment of an annelid; it bears the mouth.
peritoneum(Gr. peritonaios,, stretched around). The membrane that lines the coelom and covers the coelomic viscera.
permeaseA transporter molecule; a molecule in the cell membrane that makes it possible for another molecule (to which the membrane is not otherwise permeable) to be transported across the membrane, that is, mediated transport.
petaloids(Gr. petalon, leaf, + eidos, form). Describes flowerlike arrangement of respiratory podia in irregular sea urchins.
pH(potential of hydrogen). A symbol referring to the relative concentration of hydrogen ions in a solution; pH values are from 0 to 14, and the lower the value, the more acid or hydrogen ions in the solution. Equal to the negative logarithm of the hydrogen ion concentration.
phagocyte(Gr. phagein, to eat, + kytos, hollow vessel). Any cell that engulfs and devours microorganisms or other particles.
phagocytosis(Gr. phagein, to eat, + kytos, hollow vessel). The engulfment of a particle by a phagocyte or a protozoan.
phagosome(Gr. phagein, to eat, + soma, body). Membrane-bound vesicle in cytoplasm containing food material engulfed by phagocytosis.
phagotroph(Gr. phagein, to eat, + trophe, food). A heterotrophic organism that ingests solid particles for food.
pharynx, pl. pharynges (Gr. pharynx, gullet). The part of the digestive tract between the mouth cavity and the esophagus that, in vertebrates, is common to both digestive and respiratory tracts. In cephalochordates the gill slits open from it.
phasmid(Gr. phasma, apparition, phantom, + id). One of a pair of glands or sensory structures found in the posterior end of certain nematodes.
phenetic(Gr. phaneros, visible, evident). Refers to the use of a criterion of overall similarity to classify organisms into taxa; contrasts with classifications based explicitly on a reconstruction of phylogeny.
phenotype(Gr. phainein, to show). The visible or expressed characteristics of an organism, controlled by the genotype, but not all genes in the genotype are expressed.
phenotypic gradualismThe hypothesis that new traits, even those that are strikingly different from ancestral ones, evolve by a long series of small, incremental steps.
pheromone(Gr. pherein, to carry, + hormon, exciting, stirring up). Chemical substance released by one organism that influences the behavior or physiological processes of another organism.
phosphagen(phosphate + gen). A term for creatine phosphate and arginine phosphate, which store and may be sources of high-energy phosphate bonds.
phosphatide(phosphate + ide). A lipid with phosphorus, such as lecithin. A complex phosphoric ester lipid, such as lecithin, found in all cells. Phospholipid.
phosphorylationThe addition of a phosphate group, that is, -PO3, to a compound.
photoautotroph(Gr. photos, light, + autos, self, + trophos, feeder). An organism requiring light as a source of energy for making organic nutrients from inorganic raw materials.
photosynthesis(Gr. phos, light, + synthesis, action or putting together). The synthesis of carbohydrates from carbon dioxide and water in chlorophyll-containing cells exposed to light.
phototaxis(Gr. phos, light, + taxis, arranging, order). A taxis in which light is the orienting stimulus. An involuntary tendency for an organism to turn toward (positive) or away from (negative) light.
phototrophs(Gr. phos, photos, light, + trophe, nourishment). Organisms capable of using CO2 in the presence of light as a source of metabolic energy.
phyletic gradualismA model of evolution in which morphological evolutionary change is continuous and incremental and occurs mainly within unbranched species or lineages over long periods of geological time; contrasts with punctuated equilibrium
phyllopodium(Gr. phyllon, leaf, + pous, podos, foot). Leaflike swimming appendage of branchiopod crustaceans.
phylogenetic species conceptAn irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent.
phylogenetic systematicsSee cladistics
phylogeny(Gr. phylon, tribe, race, + geneia, origin). The origin and diversification of any taxon, or the evolutionary history of its origin and diversification, usually presented in the form of a dendrogram.
phylum, pl. phyla (N.L. from Gr. phylon, race, tribe). A chief category, between kingdom and class, of taxonomic classifications into which are grouped organisms of common descent that share a fundamental pattern of organization.
physiology(L. physiologia, natural science). A branch of biology dealing with the organic processes and phenomena of an organism or any of its parts or of a particular bodily process.
phytoflagellatesMembers of the class Phytomastigophorea, plantlike flagellates.
phytophagous(Gr. phyton, plant, + phagein, to eat). Organisms that feed on plants.
pilidium(Gr. pilidion, dim. of pilos, felt cap). Free-swimming, hat-shaped larva of nemertine worms.
pinacocyte(Gr. pinax, tablet, + kytos, hollow vessel). Flattened cells composing dermal epithelium in sponges.
pinacoderm(Gr. pinax, plank, tablet, + derma, skin). The layer of pinacocytes in sponges.
pinna(L. feather, sharp point). The external ear. Also a feather, wing, or fin or similar part.
pinocytosis(Gr. pinein, to drink, + kytos, hollow vessel, + osis, condition). Taking up of fluid by endocytosis; cell drinking.
placenta(L. flat cake). The vascular structure, embryonic and maternal, through which the embryo and fetus are nourished while in the uterus.
placentotrophy(L. placenta, flat cake, + trophos, one who feeds). Nutrition of an embryo from a placenta.
placode(Gr. plakos, flat round plate). Localized, plate-like thickening of vertebrate head ectoderm from which a specialized structure develops; such structures include eye lens, special sense organs, and certain neurons.
placoderms(Gr. plax, plate, + derma, skin). A group of heavily armored jawed fishes of the Lower Devonian to Lower Carboniferous.
placoid scale(Gr. plax, plakos, tablet, plate). Type of scale found in cartilaginous fishes, with basal plate of dentin embedded in the skin and a backwardpointing spine tipped with enamel.
plankton(Gr. neuter of planktos, wandering). The passively floating animal and plant life of a body of water; compares with nekton
plantigrade(L. planta, sole, + gradus, step, degree). Pertaining to animals that walk on the whole surface of the foot (for example, humans and bears); compares with digitigrade
planula(N.L. dim. from L. planus, flat). Free-swimming, ciliated larval type of cnidarians; usually flattened and ovoid, with an outer layer of ectodermal cells and an inner mass of endodermal cells.
planuloid ancestor(L. planus, flat, + Gr. eidos, form). Hypothetical form representing ancestor of Cnidaria and Platyhelminthes.
plasma cell(Gr. plasma, a form, mold). A descendant cell of a B cell, functions to secrete antibodies.
plasma membrane(Gr. plasma, a form, mold). A living, external, limiting, protoplasmic structure that functions to regulate exchange of nutrients across the cell surface.
plasmalemma(Gr. plasma, a form, mold, + lemma, rind, sheath). The cell membrane.
plasmid(Gr. plasma, a form, mold). A small circle of DNA that may be carried by a bacterium in addition to its genomic DNA.
plasmodium(Gr. plasma, a form, mold, + eidos, form). Multinucleate ameboid mass, syncytial.
plastid(Gr. plast, formed, molded, + L. id, feminine stem for particle of specified kind). A membranous organelle in plant cells functioning in photosynthesis and/or nutrient storage, for example, chloroplast.
plastron(Fr. plastron, breast plate). Ventral body shield of turtles; structure in corresponding position in certain arthropods; thin film of gas retained by epicuticle hairs of aquatic insects.
platelet(Gr. dim. of plattus, flat). A tiny, incomplete cell in the blood that releases substances initiating blood clotting.
pleiotropic(Gr. pleion, more, + tropos, to turn). Pertaining to a gene producing more than one effect; affecting multiple phenotypic characteristics.
pleopod(Gr. plein, to sail, + pous, podos, foot). One of the swimming appendages on the abdomen of a crustacean.
plesiomorphicAn ancestral condition of a variable character.
pleura(Gr. side, rib). The membrane that lines each half of the thorax and covers the lungs.
plexus(L. network, braid). A network, especially of nerves or blood vessels.
pluteus, pl. plutei (L. pluteus, movable shed, reading desk). Echinoid or ophiuroid larva with elongated processes like the supports of a desk; originally called "painter's easel larva".
pneumostome(Gr. pneuma, breathing, + stoma, mouth). The opening of the mantle cavity (lung) of pulmonate gastropods to the outside.
podium(Gr. pous, podos, foot). A footlike structure, for example, the tube foot of echinoderms.
poikilothermic(Gr. poikilos, variable, + thermal). Pertaining to animals whose body temperature is variable and fluctuates with that of the environment; cold blooded; compares with ectothermic
polarity(Gr. polos, axis). In systematics, the ordering of alternative states of a taxonomic character from evolutionarily ancestral to derived conditions. In developmental biology, the tendency for the axis of an ovum to orient corresponding to the axis of the mother. Also, condition of having opposite poles; differential distribution of gradation along an axis.
polarization(L. polaris, polar, + Gr. iz, make). The arrangement of positive electrical charges on one side of a surface membrane and negative electrical charges on the other side (in nerves and muscles).
Polian vesicles(from G. S. Poli, Italian naturalist). Vesicles opening into ring canal in most asteroids and holothuroids.
polyandry(Gr. polys, many, + aner, man). Condition of having more than one male mate at one time.
polygamy(Gr. polys, many, + gamos, marriage). Condition of having more than one mate at a time.
polygenic inheritanceInheritance of traits influenced by multiple alleles; traits show continuous variation between extremes; offspring are usually intermediate between the two parents; also known as blending and quantitative inheritance
polygyny(Gr. polys, many, + gyne, woman). Condition of having more than one female mate at one time.
polymer(Gr. polys, many, + meros, part). A chemical compound composed of repeated structural units called monomers.
polymerase chain reaction (PCR)A technique for preparing large quantities of DNA from tiny samples, making it easy to clone a specific gene as long as part of the sequence of the gene is known.
polymerizationThe process of forming a polymer or polymeric compound.
polymorphism(Gr. polys, many, + morphe, form). The presence in a species of more than one structural type of individual.
polynucleotide(poly + nucleotide): A nucleotide of many mononucleotides combined.
polyp(Gr. polypous, many-footed). Individual of the phylum Cnidaria, generally adapted for attachment to the substratum at the aboral end, often form colonies.
polypeptide(Gr. polys, many, + peptein, to digest). A molecule consisting of many joined amino acids, not as complex as a protein.
polyphyly(Gr. polys, many, + phylon, tribe). The condition that a taxon or other group of organisms does not contain the most recent common ancestor of all members of the group, implying that it has multiple evolutionary origins; such groups are not valid as formal taxa and are recognized as such only through error. Contrasts with monophyly and paraphyly
polyphyodont(Gr. polyphyes, manifold, + odous, tooth). Having several sets of teeth in succession.
polypide(L. polypus, polyp). An individual or zooid in a colony, specifically in ectoprocts, which has a lophophore,digestive tract, muscles, and nerve centers.
polyploid(Gr. polys, many, + ploidy, number of chromosomes). An organism possessing more than two full homologous sets of chromosomes.
polysaccharide(Gr. polys, many, + sakcharon, sugar, from Sanskrit sarkara, gravel, sugar). A carbohydrate composed of many monosaccharide units, for example, glycogen, starch, and cellulose.
polysome(polyribosome) (Gr. polys, many, + soma, body). Two or more ribosomes connected by a molecule of messenger RNA.
polytene chromosomes(Gr. polys, many, + tainia, band). Chromosomes in the somatic cells of some insects in which the chromatin replicates repeatedly without undergoing mitosis.
polyzoic(Gr. polys, many, + zoon, animal). A tapeworm forming a strobila of several to many proglottids; also, a colony of many zooids.
pongid(L. Pongo, type genus of orangutan). Of or relating to the primate family Pongidae, comprising the anthropoid apes (gorillas, chimpanzees, gibbons, orangutans).
population(L. populus, people). A group of organisms of the same species inhabiting a specific geographical locality.
populational gradualismThe observation that new genetic variants become established in a population by increasing their frequencies across generations incrementally, initially from one or a few individuals and eventually characterizing a majority of the population.
porocyte(Gr. porus; passage, pore, + kytos, hollow vessel). Type of cell found in asconoid sponges through which water enters the spongocoel.
portal system(L. porta, gate). System of large veins beginning and ending with a bed of capillaries; for example, hepatic portal and renal portal system in vertebrates.
posterior(L. latter). Situated at or toward the rear of the body; situated toward the back; in human anatomy the upright posture makes posterior and dorsal identical.
potocytosis(Gr. potos, a drinking, + kytos, hollow vessel). Endocytosis of certain small molecules and ions bound to specific receptors limited to small areas on the cell surface. The areas of the receptors are invaginated and pinch off to form tiny vesicles. See caveolae
preadaptationThe possession of a trait that coincidentally predisposes an organism for survival in an environment different from those encountered in its evolutionary history.
prebiotic synthesisThe chemical synthesis that occurred before the emergence of life.
precocial(L. praecoquere, to ripen beforehand). Referring (especially) to birds whose young are covered with down and are able to run about when newly hatched.
predaceous, predacious(L. praedator, a plunderer, praeda, prey). Living by killing and consuming other animals; predatory.
predator(L. praedator, a plunderer, praeda, prey). An organism that preys on other organisms for its food.
prehensile(L. prehendere, to seize). Adapted for grasping.
premunitionA resistance to reinfection by an animal (host) when some infective organisms remain in the host's body.
primary bilateral symmetryUsually applied to a radially symmetrical organism descended from a bilateral ancestor and developing from a bilaterally symmetrical larva.
primary radial symmetryUsually applied to a radially symmetrical organism that did not have a bilateral ancestor or larva, in contrast to a secondarily radial organism.
primate(L. primus, first). Any mammal of the order Primates, which includes the tarsiers, lemurs, marmosets, monkeys, apes, and humans.
primitive(L. primus, first). Primordial; ancient; little evolved; said of characteristics closely approximating those possessed by early ancestral types.
proboscis(Gr. pro, before, + boskein, feed). A snout or trunk. Also, tubular sucking or feeding organ with the mouth at the end as in planarians, leeches,and insects. Also, the sensory and defensive organ at the anterior end of certain invertebrates.
producers(L. producere, to bring forth). Organisms, such as plants, able to produce their own food from inorganic substances.
productionIn ecology, the energy accumulated by an organism that becomes incorporated into new biomass.
progesterone(L. pro, before, + gestare, to carry). Hormone secreted by the corpus luteum and the placenta; prepares the uterus for the fertilized egg and maintains the capacity of the uterus to hold the embryo and fetus.
proglottid(Gr. proglottis, tongue tip, from pro, before, + glotta, tongue, + id, suffix). Portion of a tapeworm containing a set of reproductive organs; usually corresponds to a segment.
prohormone(Gr. pro, before, + hormaein, to excite). A precursor of a hormone, especially a peptide hormone.
prokaryoticprocaryotic (Gr. pro, before, + karyon, kernel, nut). Not having a membrane-bound nucleus or nuclei. Prokaryotic cells characterize the bacteria and cyanobacteria.
promoterA region of DNA to which the RNA polymerase must have access for transcription of a structural gene to begin.
pronephros(Gr. pro, before, + nephros, kidney). Most anterior of three pairs of embryonic renal organs of vertebrates, functional only in adult hagfishes and larval fishes and amphibians, and vestigial in mammalian embryos. Adj, pronephric
proprioceptor(L. proprius, own, particular, + receptor). Sensory receptor located deep within the tissues, especially muscles, tendons, and joints, that is responsive to changes in muscle stretch, body position, and movement.
prosimian(Gr. pro, before, + L. simia, ape). Any member of a group of arboreal primates including lemurs, tarsiers, and lorises, but excluding monkeys, apes, and humans.
prosoma(Gr. pro, before, + soma, body). Anterior part of an invertebrate in which primitive segmentation is not visible; fused head and thorax of arthropod; cephalothorax.
prosopyle(Gr. proso, forward, + pyle, gate). Connections between the incurrent and radial canals in some sponges.
prostaglandinsA family of fatty-acid hormones, originally discovered in semen, known to have powerful effects on smooth muscle, nerves, circulation, and reproductive organs.
prostomium(Gr. protos, first, + stoma, mouth, + -idion, dim. ending). Anterior closure of a metameric animal, anterior to the mouth.
protandrous(Gr. protos, first, + aner, male). Condition of hermaphroditic animals and plants in which male organs and their products appear before the corresponding female organs and products, thus preventing self-fertilization.
protease(Gr. protein, + ase, enzyme). An enzyme that digests proteins; includes proteinases and peptidases.
protein(Gr. protein, from proteios, primary). A macromolecule of carbon, hydrogen, oxygen, and nitrogen and sometimes sulfur and phosphorus; composed of chains of amino acids joined by peptide bonds; present in all cells.
prothoracic glandsGlands in the prothorax of insects that secrete the hormone ecdysone.
prothoracicotropic hormoneSee ecdysiotropin
prothrombin(Gr. pro, before, + thrombos, clot). A constituent of blood plasma that is changed to thrombin by a catalytic sequence that includes thromboplastin, calcium, and plasma globulins; involved in blood clotting.
protist(Gr. protos, first). A member of the kingdom Protista, generally considered to include the protozoa and eukaryotic algae.
protocoel(Gr. protos, first, + koilos, hollow). The anterior coelomic compartment in some deuterostomes, corresponds to the axocoel in echinoderms.
protocooperationA mutually beneficial interaction between organisms in which the interaction is not physiologically necessary to the survival of either.
protonA subatomic particle with a positive electrical charge and having a mass of 1836 times that of an electron; found in the nucleus of atoms.
protonephridium(Gr. protos, first, + nephros, kidney). Primitive osmoregulatory or excretory organ consisting of a tubule terminating internally with flame bulb or solenocyte; the unit of a flame bulb system.
protoplasm(Gr. protos, first, + plasma, form). Organized living substance; cytoplasm and nucleoplasm of the cell.
protopod, protopodite(Gr. protos, first, + pous, podos, foot). Basal portion of crustacean appendage, containing coxa and basis.
Protostomia(Gr. protos, first, + stoma, mouth). A group of phyla in which cleavage is determinate, the coelom (in coelomate forms) is formed by proliferation of mesodermal bands (schizocoelic formation), the mesoderm is formed from a particular blastomere (called 4d), and the mouth is derived from or near the blastopore. Includes the Annelida, Arthropoda, Mollusca, and a number of minor phyla. Compares with Deuterostomia
proventriculus(L. pro, before, + ventriculum, ventricle). In birds the glandular stomach between the crop and gizzard. In insects, a muscular dilation of foregut armed internally with chitinous teeth.
proximal(L. proximus, nearest). Situated toward or near the point of attachment; opposite of distal, distant.
proximate cause(L. proximus, nearest, + causa). The factors that underlie the functioning of a biological system at a particular place and time, including those responsible for metabolic, physiological, and behavioral functions at the molecular, cellular, organismal, and population levels.
pseudocoel(Gr. pseudes, false, + koiloma, cavity). A body cavity not lined with peritoneum and not a part of the blood or digestive systems, embryonically derived from the blastocoel.
pseudopodium(Gr. pseudes, false, + podion, small foot, + eidos, form). A temporary cytoplasmic protrusion extended out from a protozoan or ameboid cell, and serving for locomotion or for taking up food.
puffStrands of DNA spread apart at certain locations on giant chromosomes of some flies where that DNA is being transcribed.
pulmonary(L. pulmo, lung, + aria, suffix denoting connected to). Relating to or associated with lungs.
punctuated equilibriumA model of evolution in which morphological evolutionary change is discontinuous, being associated primarily with discrete, geologically instantaneous events of speciation leading to phylogenetic branching; morphological evolutionary stasis characterizes species between episodes of speciation; contrasts with phyletic gradualism
pupa(L. girl, doll, puppet). Inactive quiescent stage of the holometabolous insects. It follows the larval stages and precedes the adult stage.
purine(L. purus, pure, + urina, urine). Organic base with carbon and nitrogen atoms in two interlocking rings. The parent substance of adenine, guanine, and other naturally occurring bases.
pygidium(Gr. pyge, rump, buttocks, + -idion, dim. ending). Posterior closure of a metameric animal, bearing the anus.
pyrimidine(alter. of pyridine, from Gr. pyr, fire, + id, adj. suffix, + ine). An organic base composed of a single ring of carbon and nitrogen atoms; parent substance of several bases found in nucleic acids.
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